Rotating machines applicable to motors or generators have some variations. In particular, the rotating machines applicable to generators may include a type using coil or a prototype using permanent magnet in order to generate magnetic field.
Although these two types have both merits and demerits, if putting weight on power generation efficiency, the permanent magnet rotating machine using permanent magnets to generate a magnetic field is used. The reason for this is that, in the case where two types of generators are compared with each other by size, the generator using the rotating machine using permanent magnets to generate a magnetic field can generate a stronger magnetic field than the generator using the rotating machine using coils, and the amount of magnetic flux interlinked with an armature coil increases, so that an induced voltage can be made high.
Also, the permanent magnet rotating machine is structurally categorized into a radial gap type and an axial gap type. In the radial gap type, a plurality of magnets having the radial magnetization direction are arranged in the circumferential direction of a cylindrical rotor, and coils are arranged on a cylindrical stator provided on the outer periphery side or the inner periphery side of the rotor so as to face the permanent magnets. Generally in the radial gap type, the individual stator coils are wound around an iron core having a plurality of teeth, so that magnetic fluxes from the rotor poles can efficiently link with the coils. However, this iron core produces cogging torque due to a magnetic attraction force between the magnetic pole and the iron core, which poses a problem of increased starting torque, for example, when the rotating machine is used as a generator. Also, in the general radial gap type, since the magnetic poles are arranged only in the outer peripheral part of a cylinder, there also arises a problem in that a space inside the cylinder is not utilized effectively.
On the other hand, the axial gap type has a construction such that, as shown in FIG. 13, disk-shaped rotors 103 are attached to a rotating shaft 102, and a stator 105 is provided in the direction along the rotation axis so as to face to the rotors 103. Each of the rotors 103 is provided with a plurality of permanent magnets 104, and a plurality of coils 106 are attached on the stator 105 so as to face to the permanent magnets 104. As shown in FIG. 14, the configuration sandwiching the stator between the two rotors improves the magnetic efficiency, and affords the sufficient output without the insertion of an iron core into the call. The type in which no iron core is provided in the coil, which is hereinafter referred to as a core less type, does not generate a magnetic attraction force caused by the magnetic field generated by the permanent magnets due to lack of an iron core. Therefore, this type can start the rotation of the generator with relatively small starting torque, and is advantageous for the use of a wind power generation. JP2002-320364A discloses one example of the axial gap type generator.
The axial-gap permanent magnet rotating machine shown in FIGS. 13 and 14 has eight magnetic poles and six coils. This shape can be used as a relatively small-size generator.
When the scale-up is intended, the outside diameter may be increased, or the number of stages in the direction along the rotation axis may be increased.
In the case where the outside diameter is simply increased, the magnet size may become large. A generally used permanent magnet is so-called a ferrite magnet or a rare-earth magnet. Because of its high magnetic properties, a rare-earth sintered magnet has been used in many cases. The rare-earth sintered magnet is manufactured by using a process in which the metal powder of rare-earth magnet is press molded and sintered. Therefore, a magnet larger than the critical size becomes difficult to manufacture. Further, the magnet is made generate a magnetic force through a magnetizing process in which a strong magnetic field is applied. As for the magnetization as well, it is difficult to magnetize a too large magnet at one time due to the restriction of an electromagnet for generating an external magnetic field. In the case where the fan-shaped magnet 104 as shown in FIG. 13 is made larger in size, as shown in FIGS. 15 and 16, the magnet pieces 104a and 104b are manufactured by dividing the magnet 104 into magnetizable magnet sizes, and are assembled on the rotor 103 with the like poles being adhered closely to each other. However, it is difficult to assemble the magnetized magnets because the like poles have a repulsive magnetic force.
In application for a wind power generation, the rotating machine is placed outdoors, and subject to a large temperature difference between summer and winter seasons. In the case where the permanent magnets and the rotor are jointed by bonding, a stress occurs on the bonding surface and the bonding is destroyed due to a difference in thermal expansion between the permanent magnets and the rotor because an Nd magnet has a thermal expansion coefficient in the non-magnetization direction of −1.7×10−6 [1/K] whereas the soft iron used for the rotor has a thermal expansion coefficient of 10×10−6 [1/K]. Since the stress at the edge of the magnet increases as the bonding area becomes large, the size of magnet to be fixed by bonding depends on the operation environment and the material used.
Accordingly, it is difficult to manufacture large magnets and attach them to the rotor disk in consideration of the scale-up of the axial gap type rotating machine.    Patent Document 1: JP2002-320364A